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Comment by dhosek | Hacker News Reader

I remember having a hand-held Garmin GPS unit back in the '00s that I used on a trip to Berkeley from Los Angeles (one bonus was that the speedometer in my car didn’t work¹ so I was able to use it as a speedometer for the trip). The location that it showed on the map was consistently off by about 50 feet. I wonder now if this was a tectonic drift or just an inherent inaccuracy of the unit.

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1. When I think back to that car, it’s amazing what a piece of junk it was. On the other hand, it was a convertible, so a lot of fun although my wife considered it a deathtrap.

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The natural (or artificial) inaccuracy of GPS tends to present as an offset which remains relatively stable for many minutes.

About 10 years ago, I did a lot of canyoning/bushwalking in the Blue Mountains just outside of Sydney Australia. I’d have my Garmin GPSMAP switched on for the entire journey including the car trip. For the 20+ journeys, there would be overlapping paths where I was driving the highway through the mountains. Except none of the paths overlap. They were all smooth paths which matched the curvature of the road, but each one had a different offset, typically around 2–5 metres away from the map data. (The median of these 20+ paths was highly consistent with the map data.)

Back in the day GPS was intentionally offset so countries couldn't use it for weapons.. well except it's owner of course!

It seems like the intentional degradation didn't work as intended in the first place and the inaccuracy could be corrected with little effort - at least for someone who had the resources to use it for weapons systems, so it only affected civilian use cases.

> Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS.

https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

Yes, differential GPS was used to correct the error. It wasn't useful for military purposes though: it took a LOT of time to set up a base station and allow it to collect readings and measure the SA error values. It wasn't a quick thing. And, you had to know exactly where the base station was in the first place.

If you're a civilian doing surveying work, it was fine: you stick the base station at a known location, let it work for a few hours, and then you had high-precision GPS in that area. If you're a military power attacking someone on their own ground, it's not so useful. Even worse if you're at sea.

related: a market summarizing what I know about China's intentional map offset laws, specifically on whether Google will continue following this (only 4 bettors, not very meaningful yet) https://manifold.markets/Ernie/will-google-maps-stop-followi...

Owning a junk car is a rite of passage to adulthood hood. You learn what to do when a car overheats, how to change a tire, replace a fuse, recognize the difference between burning oil and radiator fluid, how to jump start a car, and how to call a tow truck.

Important life skills

50 ft seems a little large for plate tectonics , plates move on the order of a centimeter per year [1]. (similar to fingernail growth!) [1] https://oceanservice.noaa.gov/facts/tectonics.html

I believe North America/ Australia is off by around 10 feet since GPS became a thing.

Australia's drift was taking into account and standards were updated back in 2020. Since 1994 the entire continent had moved 1.8 meters north-east.

https://www.icsm.gov.au/gda2020/what-changing-and-why

I'm sure the plate has drifted since the 2020 update and another update deployed when appropriate.

The 2011 Tohoku earthquake moved Japan's main island (Honshu) by 2.4m (8ft) in 6 minutes.

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